Numerous studies indicate that women and men differ in prevalence of pain disorders or pain intensity, possibly due to sexually dimorphic differences in detection, processing or responses to noxious stimuli. Here, we propose to study a peripheral sexually dimorphic pain mechanism that occurs in humans. The importance of this complex problem has been emphasized by recent NIH policies (NOT OD 15-102). The objective here is to determine the effects of serotonin (5HT), applied to dental pulp biopsies from women versus men, on activation of capsaicin-sensitive nociceptors, and the mechanisms mediating this response. Our central hypothesis is that 5-HT preferentially releases complement peptides C3a or C5a from peripheral tissues of women compared to men, leading to a sexually dimorphic increase in TRPV1 activities in trigeminal (TG) sensory neurons. This central hypothesis is based on substantial novel preliminary data demonstrating that 5HT produces a sexually dimorphic difference in capsaicin activation of human peptidergic fibers via release of complement peptides. The Aims will: Specific Aim #1: Determine the cell type expressing C3a, C5a, C3aR, & C5aR in female versus male human dental pulp. Additional studies will determine the effects of inflammation (irreversible pulpitis) on expression and release of C3a and C5a from female and male human tissues. Specific Aim #2: Determine the 5-HT receptor subtype(s) and G-protein and effector signaling pathways mediating 5-HT-evoked release of C3a and C5a from female and male human tissues. Specific Aim #3: Determine the receptors, G-protein and effector signaling pathways mediating C3a-and C5a-evoked increase in activities of capsaicin-sensitive neurons. The central hypothesis is highly innovative and, if supported, would have an important positive impact on the field since it supports a new model for sexually dimorphic pain mechanisms with therapeutic implications. Moreover, the use of isolated human tissue biopsies and primary neuronal cultures fosters studies on the cellular mechanisms mediating this sexually dimorphic effect and increases translational significance.